Rendered image is not as smooth as in 3d preview
Well, Cegaton already found the problem, but the question needs an answer for future users.

The Subdivision surface modifier has two important parameters: the view and the render subdivisions. As their name suggests, view determines how smoothed out the surface will be when viewing it in the viewport, and render does the same for rendered images.


This is useful if you want a smoother object when rendering, but want a better performance of the viewport and don't need much detail when previewing – you can set a lower view and a higher render.

If you set the render parameter lower than the view parameter, objects will appear smoother in the viewport than in actual renders.

Another possible cause of this problem is that the modifier is visible  but isn't being rendered . In that case, click the camera icon to enable rendering of the modifier (in the image above, both rendering and viewing are enabled).